The present invention generally relates to relaxation oscillators, and, more particularly, to a relaxation oscillator with a self-biased comparator.
Relaxation oscillators are widely used in modern electronic systems including radios, telecommunications, computers, and other electronic systems to generate oscillator signals. The oscillator signals are required to meet timing critical requirements such as modulation and demodulation of message signals in communication systems, synchronous operation of electronic circuits, and so forth. A conventional relaxation oscillator includes a resistor-capacitor (RC) circuit connected to a power supply, first and second comparators, and a logic circuit. The RC circuit includes a resistor and first and second capacitors. The first and second capacitors are connected to the first and second comparators to provide first and second capacitor voltages to negative terminals of the first and second comparators, respectively. The first and second comparators receive a threshold voltage generated by a resistor-divider circuit connected between a power supply and ground or receive a suitably divided bandgap reference voltage. The first and second capacitors are alternately charged by the power supply by way of the resistor. When a capacitor voltage corresponding to the first capacitor reaches the threshold voltage, the corresponding comparator that receives the capacitor voltage trips and causes a transition in an output signal generated by the comparator. Subsequently, the first capacitor discharges and simultaneously the second capacitor is charged until its capacitor voltage reaches the threshold voltage. Thereafter, the charging and discharging of the first and second capacitors is repeated. The logic circuit is connected to outputs of the first and second comparators and generates an oscillator signal based on transitions in output signals generated by the first and second comparators and control signals for charging and discharging the first and second capacitors.
The first and second comparators require a current source to operate, which occupies silicon area and consumes power. Since cost and battery life of electronic devices have placed strict constraints on system-on-a-chip (SoC) area and power consumption, having separate circuits for the first and second comparators and the current source considerably increases the area and power consumed by the SoC. Additionally, propagation delays of the first and second comparators vary substantially across process corners, which leads to a variation in the frequency of the oscillator signal and deterioration in the fidelity of the oscillator signal.
Therefore, it would be advantageous to have a relaxation oscillator that has low frequency spread across low supply voltage and process and temperature corners, that consumes less power, has a small area footprint, and that overcomes the above-mentioned limitations of existing relaxation oscillators.